Morphological responses of ostracods to heavy metal contamination: freshwater ostracods as bioindicators of pollution in Ichkeul Lake and rivers ecosystem, northern Tunisia

The analysis of the ostracofauna at the studied sites proves to be a reliable and cost-effective environmental tool for public managers or those responsible for the conservation of the Ichkeul ecosystem. In addition, all these characteristics are clearly applicable to the reconstruction of the history of anthropogenic impacts on polluted environments. Cyprideis torosa, which was found to be dominant species in our study, is recognized for its opportunistic and competitive nature, enabling it to colonize polluted areas with high levels of metallic trace elements. The unpolluted site (S3) at the mouth of the Sejnane river which flows into Ichkeul Lake revealed the presence of glassy species with normal morphological valves, while the polluted sites contained ostracods marked by white-beige or orange color. The Energy-dispersive X-ray Spectroscopy (EDX) microanalysis of ostracod shells using a transmission electron microscope revealed iron and copper concentrations at almost all studied sites. Iron is responsible for the red–orange (rust) color of the studied shells which exhibited morphological abnormalities. Indeed, the color observed in the studied ostracods could be attributed to the presence of iron. Microanalysis performed on the investigated ostracods’ shells reveals the effects of Jebel Ichkeul structure. Thus, the Jurassic and Triassic metamorphic series, coupled with lode mineralization and iron-rich hydrothermal activity, have influenced the shape and the color of these shells. Furthermore, the ostracod test contains numerous elements that do not typically fit into the carbonate mineral network. The absence of ostracofauna from the river that drains directly from the Jalta mine site (S13) confirms the anthropogenic perturbation and the metal pollution affecting the natural composition of the waters ecosystems. Nevertheless, more sophisticated studies are necessary to quantify the metals stored in organisms’ molts and to investigate their bio-availability. This study also highlights the results of the summer period, compared to those of the winter period already published (Ouchir et al., in Arab J Geosci 9(9):1–12, 2016), revealing an accumulation of metallic elements attributed to the combined effects of water and wind transport agents, which can mobilize the highly mineralized fine fractions, particularly during the summer.